We are looking for an op-amp to replace the input stage of a data acquisition instrument. Currently the front-end is an AD8510, configured as a non-inverting buffer. This frontend shall now be expanded with a 10:1 resistor divider to allow measuring higher input voltages. This attenuator however increases the source impedance as seen by the AD8510 to ~100kohms. We noticed a significant increase in THD:
A 10Vpk 1kHz sinewave connected without the attenuator (low source resistance, about 5k ohms) gives -105dB THD for the complete system, but the same signal connected via the attenuator results in -65dB THD. I assume this is caused by modulation of the op-amp input capacitance, which has no ill effect as long as the source resistance is small, but not so with a high source resistance.
Can you recommend a replacement for the AD8510 with low common mode distortion? The system constraints are:
single device
MSOP8 package
+/-15V operation
CM input range V+ - 2V, V- +2V or better
low voltage and current noise
low voltage and bias current offset
supply current less than 3mA
Many thanks, AK
